Submarine evidence of a debris avalanche deposit on the eastern slope of Santorini volcano, Greece Katherine Lynn Croff Bell a, ⁎, Steven N. Carey a , Paraskevi Nomikou b, c , Haraldur Sigurdsson a , Dimitris Sakellariou c a Graduate School of Oceanography, University of Rhode Island, South Ferry Road, Narragansett, RI, USA b National Kapodistrian University of Athens, 15784 Zografou, Athens, Greece c Hellenic Center for Marine Research, Institute of Oceanography, Anavyssos, Greece abstract article info Article history: Received 3 May 2011 Received in revised form 5 April 2012 Accepted 7 May 2012 Available online 22 May 2012 Keywords: South Aegean Sea Santorini volcano Submarine Debris avalanche Minoan eruption Hummocky seafloor features were discovered on the eastern flank of Santorini volcano, Greece. Multibeam bathymetric mapping, airgun seismic profiling, side scan sonar survey, and remotely operated vehicle (ROV) dives have been carried out to characterize the nature of the hummocks. These hummocks appear to be com- posed of several tens of blocks that are up to several hundred meters in diameter, and are the surface expression of a much larger deposit than is observed in the bathymetry. The sidescan and airgun data show that the deposit covers an area of approximately 6 km wide by 20 km long, and is up to 75 m thick. We estimate the total volume of the deposit to be approximately 4.4 × 10 9 m 3 . Sampling of these blocks show they are composed of pyroclastic flow deposits produced during the Minoan eruption of Santorini (ca. 3600 BP). We propose that the deposit is the result of a multi-stage landslide event that was caused by one of the several large earthquakes or volcanic erup- tions that have occurred in the vicinity of Santorini since the Minoan eruption. One or more of these events likely triggered the destabilization of a part of the eastern flank of Santorini, which led to a debris avalanche, depositing blocks and forming a hummocky terrain at the base of the island's slope. The mass movement later evolved into a turbulent suspension flow that traveled 20 km or more from the presumed initial failure. Given the size of the landslide deposit, it might have a tsunami potentially affecting the islands across the southern Aegean Sea. The understanding of earthquake–landslide dynamics has important implications for hazard assessment in this seismically active, historical, and highly populated region of the world. © 2012 Elsevier B.V. All rights reserved. 1. Introduction Hummocky deposits were discovered on the eastern submarine slope of Santorini using multibeam bathymetric mapping and seismic profiling aboard the R/V Aegaeo in 2006 (Alexandri et al., 2001, 2006; Sigurdsson et al., 2006), and were further mapped with sidescan sonar and observed with remotely operated vehicles (ROVs) aboard R/V Endeavor in 2006 and E/V Nautilus in 2010. The hummocks are similar in morphology to debris avalanche deposits found in other volcanic arc environments, such as the Lesser Antilles, Nisyros, and the Aleutian Arc (Coombs et al., 2007; Deplus et al., 2001; Siebert, 1984; Tibaldi et al., 2008). We hypothesize that the hummocks are the result of a debris avalanche that was triggered by one or more of the several large earthquakes or volcanic eruptions that have occurred in the vicinity of Santorini over the past 3600 years (Papazachos et al., 2000b, 2007). Debris avalanches are one of the most dangerous hazards in coastal volcanic environments. The large, catastrophic movement of volcaniclastic debris has the ability to bury entire towns (Catane et al., 2008), and are able to cause tsunamis that can devastate nearby coastal communities (Tappin et al., 2001; Tinti et al., 2008; Ward and Day, 2001; Whelan and Kelletat, 2003). Studying the characteristics of past events may allow us to more fully understand how these events are triggered and the potential dynamics of future events. Recent studies have shown that the processes, magnitude, and char- acterization of submarine landslides are distinct from subaerial events (Hampton et al., 1996; Korup et al., 2007; Locat and Lee, 2002). Further- more, Canals et al. (2004) have identified the need for criteria by which underwater debris flows and debris avalanches can be classified for fur- ther study. They have developed a classification scheme that includes the assessment of: (1) morphology and dimensions of failure deposits; (2) scars and slip plane geometries; (3) run-out distances; (4) and, trig- gering mechanisms and ages of slide events (Canals et al., 2004). We aim to apply these criteria to evaluate the hummocky deposits found east of Santorini using a combination of geophysical and geological data collected in 2006 and 2010. Each of the characteristics can be eval- uated to varying degrees, depending on the data available. Tectonophysics 597–598 (2013) 147–160 ⁎ Corresponding author. Tel.: + 1 401 874 6186; fax: + 1 401 874 6223. E-mail address: croff@alum.mit.edu (K.L.C. Bell). 0040-1951/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2012.05.006 Contents lists available at SciVerse ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto